1. Field of the Invention
The present invention relates in particular to a technology for observing an underwater object with a laser imaging apparatus.
2. Description of the Related Art
A Japanese Patent Application, First Publication, H7-072250 discloses an imaging technology, applicable to underwater marine construction projects and others, for reliably identifying an object disposed in a murky underwater environment with a laser-based imaging equipment. The laser imaging technology disclosed in this publication is based on projecting a pulsed laser beam towards a distant underwater object from an observation/imaging unit, and producing images of the object by detecting the laser beam pulses reflected from the object.
When using such an laser-based underwater observation apparatus, the distance over which the object is observable (visibility) varies depending on the turbidity of the sea water (beam propagation medium). For this reason, it is necessary to decide where to locate the observation equipment, depending on the turbidity of the environment. Also, depending on the operating characteristics of the observation equipment, such as pulsing rate, pulse width and pulse energy of the laser irradiation device, the settings for ancillary devices such as zoom lens and shutter speed of the camera also need to be adjusted.
When it is desired to observe the object in its realistic state in spite of the turbid state of the underwater environment, it is preferable to capture the image in color. For such reasons, there has been strong demand for a technology to enable observation of underwater objects in color.
Furthermore, in the conventional arrangement described above, the beam pulses are delivered from a support vessel to a submersible vessel through optical fiber cable, resulting that the optical power of the output pulses is limited by the transmission capacity of the optical fiber cable. On the other hand, it is known that the clarity of the object image formed depends on the properties of the laser pulses such as the coherency of the beam manifesting in narrow width of the pulse and high optical power. Thus, the problem in the conventional laser-based observation equipment is that it has not been possible to irradiate laser pulses of sufficiently high optical power towards the object to generate clear images.